1. Field of the Invention
The present invention generally relates to an electronic circuit, and more particularly, to an electronic circuit that has a transimpedance amplifier.
2. Description of the Related Art
A transimpedance amplifier (TIA) is an amplifier circuit that converts current into voltage and amplifies the voltage. Such a transimpedance amplifier is used for optical communications, for example. On the light receiving side of an optical communication, a light receiving element such as a photodiode converts light transmitted through an optical fiber into a current. A TIA amplifies the current output from the light receiving element, and outputs the amplified current as a voltage signal. The input to the TIA includes an input current that is a DC component, and an input signal that is a high-frequency component. There is an input current range within which the TIA can amplify the current output with precision. Accordingly, when the amount of input current changes greatly, the TIA cannot amplify the input signal with high precision. Therefore, some TIAs have automatic gain control (AGC) circuits. An AGC circuit controls the gain of a TIA, based on the output voltage of the TIA. When the output voltage of the TIA is low, the AGC circuit does not control the gain. However, as the output voltage increases, the AGC circuit reduces the gain of the TIA. In this manner, an amplifier that can amplify a small input current as well as a large input current with high precision can be realized. The output current of a light receiving element is amplified by a TIA circuit equipped with an AGC circuit, so that weak optical signals as well as strong optical signals can be amplified with high precision. Japanese Unexamined Patent Publication No. 2003-258580 discloses a TIA circuit that has an AGC circuit.
In a TIA having an AGC circuit, the AGC circuit is expected to have greater effects. More specifically, the AGC circuit is expected to control transimpedance as a gain more effectively.